The present invention relates to improvements in the field of earth materials testing. More particularly, the invention is concerned with an improved method and device for determining in-situ rheological properties of earth materials.
The knowledge of rheological properties of earth materials is an essential condition for the design of structural elements in contact with soils or rocks, to which they transfer the applied loads. Typical rheological properties are the creep properties of the material and its time or rate-dependent deformation or strength. The earth materials to which the invention pertains are soils, both frozen and unfrozen, ice, and weak rocks, such as rocksalt and potash. Practical problems requiring the knowledge of rheological properties of such earth materials are, for instance, the design of foundations in frozen and unfrozen soils, the bearing capacity of ice covers, and the design of tunnel and shaft linings.
For determining the above mentioned rheological properties, both laboratory and in-situ methods are presently being used. In the former, undisturbed soil samples are taken from borings at selected levels, and are subjected to certain tests pertinent to the purpose at hand. The latter, in-situ methods do not require soil sampling, but they permit to measure only a limited number of rheological properties. Their main advantages over the former are their rapidity and ability to furnish a continuous picture of the geotechnical profile of the site.
Not considering the geophysical methods, which measure only the physical properties of the ground, principal geotechnical in-situ methods presently in use are the Cone Penetration Test (CPT), the Pressuremeter Test (PMT) and the Flat Dilatometer Test (DMT).
The CPT method is a standardized method in which a pressure-sensitive cone having a diameter of 3.56 cm and an apex angle of 60.degree., and fixed to the end of a drill rod of the same diameter, is pushed into the soil at a rate of 2 cm/sec. From the recorded cone resistance (both total and piezometric pressure), certain mechanical properties of penetrated soils can be deduced, using theoretical models and statistical correlations. Although electrical cone tests have been in geotechnical use since 1950's, such tests have been introduced also to frozen soils only in the 1970's (see Ladanyi, B., "Determination of Geotechnical Parameters of Frozen Soils by Means of the Cone Penetration Test", Proc. 2nd Europ. Symp. on Penetration Testing, Amsterdam (1982), Vol. 1, pages 671-678). The CPT method, although being based on a continuous penetration mode, requires heavy penetration equipment and furnishes only information on soil strength properties, with no data on soil deformability and on stress-strain properties.
The PMT method, introduced to geotechnical practice by Menard in the 1950's, consists in placing an inflatable probe into a predrilled (or self-drilled) borehole of the same diameter. The hole is drilled down to a certain level, and the test is made at that level by keeping the probe fixed in place. The test is performed by inflating the probe and by recording the relationship between the applied pressure, the hole enlargement and the time. For any additional testing, the hole is drilled further, and the test is performed at another fixed level. In unfrozen soils, this method has been used essentially for determining the short-term mechanical properties of soils. The theoretical interpretation of the test in ordinary soils and rocks is presently well developed. In frozen soils, the method has been used for creep properties determination since 1973 (see Ladanyi, B. and Johnston, G. M., "Evaluation of In-Situ Creep Properties of Frozen Soils with the Pressuremeter", Proc. 2nd Int. Permafrost Conf., Yakutsk, USSR, North Amer. Contribution, NAS, Washington, D.C., (1973), pages 310-318). Being based on a discontinuous penetration mode, the PMT method gives information limited only to certain previously selected levels and thus does not provide a continuous soil profile. In addition, the method requires a rather sophisticated apparatus and a skilled personnel.
In the DMT method, introduced by Marchetti in 1980 (see Marchetti, S., "In-Situ Tests by Flat D-Y61 ilatometer", J. of Geotech. Engrg. Div., ASCE, Vol. 106, No. GT3, (1980), pages 299-321), use is made of a soil testing tool ressembling a thick spade, which is pushed into the soil at the end of a drill rod. The measurement is made by slightly inflating a metallic diaphragm located at one side of the spade. The test interpretation is based exclusively on statistical correlations with soil properties deduced from other, more advanced, types of tests, and thus the information furnished is not clear and lacks theoretical background.